Oxidative Fluorination of Selenium and Tellurium Compounds using a Thermally Stable Phosphonium SF5 − Salt Accessible from SF6

Abstract Fluorinated group 16 moieties are attractive building blocks in synthetic chemistry but only few synthetic methods are available to prepare them. Herein, we report a new oxidative fluorination reagent capable of stabilizing reactive fluorinated anions. It consists of an SF5 − anion and a chemically inert phosphonium cation and is exceptionally thermally stable. Accordingly, it was used to generate the SeF5 − and TeF5 − anions from the elemental chalcogens and to prepare the unknown tetrafluoro(phenyl)‐λ5‐selenate PhSeF4 − and ‐tellurate PhTeF4 − from the corresponding diphenyl dichalcogenides. In addition, we show that further derivatization of [PhTeF4]− by oxidation to trans‐PhTeF4O− and subsequent alkylation gives access to a new class of trans‐(alkoxy)(phenyl)tetrafluoro‐λ6‐tellanes (trans‐PhTeF4OR), thus providing an approach to introduce the functional group into organic molecules.

General remarks: Unless noted otherwise, all manipulations were performed under an inert atmosphere of dry argon, using standard Schlenk and drybox techniques. Dry and oxygen-free solvents were employed. All glassware was oven-dried at 160 °C prior to use. Silica used for flash column chromatography was dried at 150 °C for 3 days. Chemical shifts are given in parts per million (ppm) relative to SiMe4 ( 1 H, 13 C, 29 Si), 85% H3PO4 ( 31 P), CCl3F ( 19 F), Me2Se ( 77 Se), 90% Me2Te in C6D6 ( 125 Te) and were referenced to the residual solvent signals (C6D6: 1 H H = 7.16, 13 C C = 128.06; CD3CN: H = 1.94, 13 C C = 118.26; CDCl3: H = 7.26, 13 C C = 77.16; CD2Cl2: H = 5.32, 13 C C = 53.84; THF-d8: 1 H H = 1.72, 13 C C = 67.21) or internally by the instrument after locking and shimming to the deuterated solvent ( 19 F, 31 P). NMR multiplicities are abbreviated as follows: s = singlet, d = doublet, t = triplet, p = pentet, sept = septet, m = multiplet, br = broad signal. Mass spectra were recorded using an Orbitrap LTQ XL (Thermo Scientific) spectrometer and an Orbitrap QExactive (Thermo Scientific) spectrometer. Elemental analyses were determined by the microanalytic laboratory of the Westfälische Wilhelms-Universität Münster. IR spectra were recorded using a Bruker ALPHA II ATR-FTIR spectrometer. Irradiation experiments were performed with a EvoluChem™ LED Typ P205-18-1 (365 nm). Safety remarks: The use of a burst shield is strongly recommended for all reactions carried out in closed Schlenk flasks under pressurized gas or when heated above the boiling point of the respective solvent! Caution should be exercised when drying 3-chloroperoxybenzoic acid (m-CPBA) as it is shock sensitive in dried form. Therefore, it is recommended to use only small amounts, not to heat the sample during the drying process and to dissolve the dried m-CPBA in a suitable solvent. Reagents and Handling: Phosphine 1 and the fluorophosphonium salt [FP(NIiPr)3][SF5] were synthesized according to literature procedures. [1,2] All other compounds were purchased from commercial sources (Sigma Aldrich, Alfa Aesar, abcr GmbH, Tokyo Chemical Industry). The amount of active 3-chloroperoxybenzoic acid (m-CPBA) in the purchased batch has been determined by iodometric titration [3] and the m-CPBA was then carefully dried in vacuo, dissolved in DCM and stored at 7 °C. Diphenyl diselenide was recrystallized from hexane and triphenyl phosphine was sublimated before use. All other chemicals were used as received.

Synthesis of the SF5salt 2
The SF5salt 2 was prepared either by heating (70 °C) or by irradiation (with light at 365 nm) of a solution of phosphine 1 (600 mg, 0.968 mmol) in THF (3 mL) under 2 bar of SF6 gas in a sealed Schlenk flask. After 12 hours, a crystalline material had been formed which was isolated by filtration and contained pure 2 as indicated by NMR analysis. The volume of the mother liquor was then reduced to 1 mL and diethyl ether (7 mL) was added. The formed precipitate was collected, washed with diethyl ether, until the supernatant solution turned colorless (5  5 mL), and dried in vacuo to afford 2 as a white crystalline solid.

Preparation of 5
The SF5salt 2 (100 mg, 0.131 mmol, 1.0 eq.) was added to a solution of Ph2Te2 (27 mg, 0.065 mmol, 0.5 eq.) in THF (5 mL). The mixture was stirred in a sealed glass vessel for 14 hours at 70 °C. The reaction mixture was then cooled to ambient temperature and the volume was reduced to 2 mL. The product was obtained as colorless crystals by slow diffusion of diethyl ether into the reaction mixture. The crystalline material was separated by filtration, washed with diethyl ether, and dried in vacuo. Note: The reaction can also be performed at ambient temperature. Full conversion of 2 (0.065 mmol) was then observed after 3 days. Yield: 92% (110 mg, 0.120 mmol). 1

Preparation of 6
An NMR tube equipped with an PTFE inlay was charged with a suspension of Se2Ph2 (10 mg, 0.033 mmol, 0.5 eq.) and 2 (51 mg, 0.066 mmol, 1.0 eq) in THF (0.3 mL). The NMR tube was sealed with a PTFE valve and it was heated at 90 °C for 48 hours. After cooling the reaction mixture to room temperature, the THF solution was carefully layered with diethyl ether (1 mL). After two days the formed crystals were isolated and washed with diethyl ether (2  1 ml) using a syringe to remove the solvent. After drying in vacuo, compounds 6 was obtained as pale yellow, crystalline solid. Note: The compound decomposes slowly in the presence of glass, as indicated by an increasing intensity of the [FHF]resonance in the 1 H NMR spectrum. The NMR measurements were therefore carried out using NMR tubes with a PTFE inlay. Yield: 60% (34 mg, 0.039 mmol).

1.6
Trapping experiments to identify the sulfur species formed in the synthesis of 3 and 5 using triphenylphosphine The trapping experiments were performed for the tellurium compounds 3 and 5 since the selenium compounds 4 and 6 react with triphenylphosphine.
After heating THF solutions of either 2 and elemental tellurium (reaction A) or of 2 and diphenyl ditelluride (reaction B) for 48 hours at 70 °C, triphenylphosphine (approx. 4 eq.) was added to the resulting reaction mixtures, respectively (Scheme S1). Quantitative 19 F and 31 P NMR measurements were performed to determine the relative amounts of difluorotriphenylphosphorane and triphenylphosphine sulfide compared to the fluorophosphonium cation of 3 and 5, respectively (Table S2). For reaction A, SPPh3 is formed stoichiometrically along with small amounts of F2PPh3, indicating that elemental sulfur is formed during the reaction and that some SF species was still present in the reaction mixture (presumably residual SF5salt). For reaction B, SPPh3 and F2PPh3 are formed in a 2:1 ratio, consistent with the formation of SSF2 during the reaction. The resonance of thiothionylfluoride [4] was detected in the 19 F NMR spectrum of the reaction mixture of 2 with diphenyl ditelluride ( Figure S32).
Scheme S1: Reaction of 2 with elemental tellurium (top, A) and of 2 with diphenyl ditelluride (bottom, B) and the subsequent addition of an excess of PPh3 to identify the "SF" species formed in the reaction.       Anhydrous m-CPBA dissolved in DCM (0.086 M, 2.56 mL, 1.1 eq.) was added and the reaction mixture was stirred for 30 minutes at ambient temperature. Quantitative oxidation of the anion was indicated by NMR spectroscopy. To remove the remaining m-CPBA and the meta-chlorobenzoic acid formed in the reaction, the following workup was carried out: The volume of the DCM solution was reduced to approximately 0.5 mL, diethyl ether (7 mL) was added, and the mixture was vigorously shaken for 60 minutes. The white precipitate was then isolated and washed with diethyl ether (8  3 mL). The precipitate was again dissolved in DCM and the whole procedure was repeated once more. Crystals of 7 were obtained by slow diffusion of diethyl ether   -21 -

1.8
Attempts of oxidizing 6 with m-CPBA Synthesis 6 (55.7 mg, 0.064 mmol, 1.0 eq.) was dissolved in THF (0.3 mL) in a PTFE vessel and a solution of anhydrous m-CPBA in DCM (0.533 M, 0.13 ml, 1.1 eq) was added. The mixture immediately turned yellow and was stirred for 10 minutes at room temperature before being analyzed by NMR spectroscopy. The 19 F{ 1 H} NMR spectrum revealed the formation of 3-chlorobenzoyl fluoride as well as of fluorobenzene, indicating that the deoxyfluorination of 3-chlorperbenzoic acid took place with degradation of 6. No resonances of a selenium species were found in the 19 F and 77 Se NMR spectra.

Synthesis of trans-alkoxytetrafluoro(phenyl)-λ 6 -telluranes 8-10
General procedure: An excess of the respective alkylation reagent (0.45 mmol, 7.0 eq., dried over 3 Å molecular sieve) was added to a solution of 7 (60 mg, 0.064 mmol, 1.0 eq.) and MeCN (2 mL) in a PTFE tube. The tube was tightly sealed, and the mixture was stirred at 70 °C for 12 hours. The solution was then allowed to cool to room temperature and transferred into a Schlenk flask. An aliquot was taken to determine the conversion using quantitative 19 F NMR spectroscopy with α,α,αtrifluorotoluene as internal standard (Table S3). Information on the work up procedure for 8-10 are given in chapter 1.9.11.9.3.

Preparation of 10
After the alkylation of 7 with benzyl bromide was carried out according to the general procedure, the volatiles were removed in vacuo. n-Hexane (10 mL) was added, and the suspension was vigorously stirred for 20 minutes. The suspension was filtered, and the solvent of the filtrate was removed in vacuo. The remaining oil was transferred onto a pad of silica and it was washed with n-hexane until the eluent contained no benzyl bromide. The product was then eluted using a mixture of DCM and n-hexane (1:3). All volatiles were then removed in vacuo. After recrystallization of 10 in a minimal amount of n-hexane (<1 mL) at ˗40 °C, it was obtained as an analytically pure white solid. Isolated yield: 28% (7 mg, 0.018 mmol).      -29 -

Preparation of 8
After the alkylation of 7 with methyl iodide was carried out according to the general procedure, the volatiles were removed in vacuo at 0 °C. After purification of the residue by column chromatography (SiO2, ethyl acetate and n-hexane (1:8), Rf = 0.51), the product 8 was obtained as a colorless oil.

Preparation of 9
After the alkylation of 7 with allyl iodide was carried out according to the general procedure, the volatiles were removed in vacuo. The residue was purified by column chromatography (SiO2, ethyl acetate and hexane (1:8), Rf = 0.56) to afford 9 as a colorless oil.  -32 -

Reaction of [FP(NsItBu)3][PhTeF4] (5) with alkylation reagents
The PhTeF5salt 5 did not react with allyl iodide but decomposed by using MeOTf or (Et3O)BF4 under formation of MeF or EtF, respectively ( Figure S58 and Figure S60) and degradation of the fluorophosphonium cation to the PF6anion ( Figure S59). The reaction with (Et3O)BF4 proceeded slowly at room temperature. After four weeks, single crystals suitable for an X-ray diffraction study were obtained from a trifluorotoluene solution containing 5 and (Et3O)BF4 in a ratio of 1:2. The XRD study revealed the molecular structure of the decomposition product 11, which could not be isolated but decomposed upon prolonged storage of the solution.

Hydrolysis experiments of [FP(NsItBu)3][PhTeF4] (5) and trans-PhTeF4(OMe) (10)
Compound 5: Degassed H2O (10 eq.) was added to an NMR tube containing 5 (30 mg, 0.03 mmol) in MeCN-d3. After 24 hours, broadening of the 19 F NMR resonance of the PhTeF4anion was observed. Over a period of 55 days the intensity of the 19 F resonance of the PhTeF4anion decreased and a brown precipitate was formed.   Compound 10: Compound 10 (8 mg, 0.02 mmol) was dissolved in a 10:1 mixture of MeCN/H2O and the solution was analyzed by 19 F NMR spectroscopy. After 3 hours, a new set of signals appeared in the 19 F NMR spectrum, indicating that the mono-hydrolysis product was formed in less than 1% (see Figure S64).

X-ray Diffraction Studies
General: Single-crystal X-ray diffraction data were collected on a Bruker AXS and on a Bruker Photon 100 detector using Mo-K radiation ( = 0.71073 Å). Crystals were selected under oil, mounted on nylon loops and then immediately placed in a cold stream of N2 on a diffractometer. The APEX2 software (for Bruker AXS) and the APEX3 software (for Bruker Photon 100) were used to operate the diffractometers. The data was integrated with SAINT15 [5] and corrected for absorption effects based on Gaussian numerical integration and scaled with SADABS.16. [6] Using Olex2, [7] the structures were solved with SIR2004 [8] using direct methods or ShelXT [9] with Intrinsic Phasing and refined with the ShelXL [10] refinement package using Least Squares minimization.

Crystal structure data of compound 2
Single crystals were obtained by slowly cooling an 80 °C hot, saturated THF solution of 2. A singlecrystal X-ray structure analysis revealed that 2 crystallizes in the monoclinic space group P21/n. The asymmetric unit contains one molecule of the cation and the anion of salt 2. The SF5 anion is partially disordered over two positions (occupancy: 59:41). Figure S69: The asymmetric unit of the crystal structure of 2 with thermal ellipsoid plot at the 50% levels of probability. Hydrogen atoms are omitted for clarity.

Crystal structure data of compound 5
Single crystals were obtained by vapor diffusion of diethyl ether into a saturated THF solution of 5. Compound 5 crystallizes in the orthorhombic space group Pbca. The asymmetric unit contains one molecule of the cation and the anion of salt 5. The fluorophosphonium cation is disordered over two positions (occupancy: 66:34). Figure S70: The asymmetric unit of the crystal structure of 5 with thermal ellipsoid plot at the 50% levels of probability. Hydrogen atoms are omitted for clarity.

Crystal structure data of compound 6
Single crystals were obtained by liquid/liquid diffusion of diethyl ether into a saturated THF solution of 6. A single-crystal X-ray structure analysis revealed that 6 crystallizes in the orthorhombic space group Pbca. The asymmetric unit contains one molecule of the cation and the anion of the salt 6. Both the P-F moiety and the backbone of two substituents in the fluorophosphonium cation are disordered over two positions (occupancy: 74:26 (P-F)/ 58:42 (Substituent A) / 71:29 (Substituent B)). Figure S71: The asymmetric unit of the crystal structure of 6 with thermal ellipsoid plot at the 50% levels of probability. Hydrogen atoms are omitted for clarity.    -44 -

Crystal structure data of compound 11
Single crystals were obtained by storing a reaction mixture of 5 and Et3O·BF4 in trifluorotoluene for four weeks. A single-crystal X-ray structure analysis revealed that 11 crystallizes in the triclinic space group P1 ̅ . The asymmetric unit contains one molecule of 11(BF4)2 and 1.5 triflourotoluene molecules. 11 as well as one trifluorotoluene molecule are disordered over two positions respectively (occupancy: 60:40 (11) / 50:50 (triflourotoluene)). -45 - Figure S74: The asymmetric unit of the crystal structure of 11 with thermal ellipsoid plot at the 50% levels of probability. Hydrogen atoms, except in the two protic imidazolidinium moieties, are omitted for clarity. The geometry optimizations and frequency calculations were performed with Gaussian 09, [11] using the wB97XD range separated hybrid functional [12] . Triple zeta basis sets were used in the calculations: aug-cc-pVTZ for C, O, F, H, [13] while selenium and tellurium were described with aug-cc-pVTZ-PP that provides additional diffuse functions and additional pseudopotentials. [14] The absence of imaginary frequencies confirmed that each optimized structure is at a local minimum (for SeF3Ph an optimized structure with one imaginary frequency of -22.4 cm -1 was used). Such low imaginary frequencies are of minor concern as they occur due to numerical instabilities of the algorithm.
To shed light on the dynamic behavior of PhSeF4in the NMR, a second conformation with one F ligand being the apex of the square pyramid rather than the phenyl ring was investigated. Structure optimization were performed with the wB97XD3 and the def2-TZVP basis set [15] in implicit THF modelled by a polarizable continuum to account for bulk solvent effects. To search for the transition state between the two conformers, the nudged electric band (NEB) method as implemented in ORCA [16] was used obtain an initial guess structure with wB97XD3/def2-TZVP/THF. The final transition state was converged by eigenvector following and identified by one imaginary frequency that coincides with the reaction coordinates. A second imaginary frequency with a very low wave number < 12 cm -1 was also found (see above).
Final single point energies of these structures were calculated coupled cluster theory in the domain based local pair-natural orbital singles-and doubled coupled cluster with perturbative triples corrections (DLPNO-CCSD(T)) [17,18] and the def2-TZVP basis set in implicitly modelled THF.
To check whether dissociation of Ffrom PhSeF4and reassociating is responsible for the observed dynamics in the NMR, the dissociation of Fwas modelled as a (hypothetical) transfer reaction to HF to yield FHFmimicking the presence of a potential acceptor.
As the dissociation of F-was monotonously uphill in electronic energy, a method established by Baik et al. was used that estimates the free energy of transition state for such reactions by taking into account the onset of translational and rotational entropy upon dissociation. [19]     As the energy difference between the two PhTeF4(OMe) (8) isomers is rather small, reference calculations with DLPNO-CCSD(T) [18,19] were performed in the gas phase and in implicit acetonitrile. Thermodynamic corrections were taken from the DFT calculations (wB97XD/augcc-pVTZ(-PP)). Results are listed in Table S11.